As a police sergeant in a major metropolitian area, I can see both good and bad things.
The good:
1) No longer a he said / she said issue. Officers can be either exonarated or held accountable for their actions.
2) It will make those officers that might be inclined to take negative actions think twice before they do so.
The bad:
1) Video has only a small field of view. There is a lot of information that is missed by cameras, whether it be things outside the visual angle or on the camera but missed due to obstructions.
2) People looking at the video and using that as the whole of the evidence. Cameras lack context. Even if it were on 24/7, there are alot of things that go into a police officer's decisions that you can't see because they are mental processes.
3) Regardless of how upstanding a police officer is, the fact of the matter is that cameras may curtail enforcement action. Because of the two issues I've listed above, the correct actions (within the context of the officer's moral intent), officers will still be penalized for their actions. Whether it happens to them, or they learn about it through the grapevine, the effect will still be the same.
Now, each jurisdiction needs to weigh the idea of any type of camera system on its own merits, based on the needs of the citizens and the department. In my agency, we are field testing lapel type cameras to determine their effectiveness. I don't know whether they will be a good thing or a bad thing. Dashcams in my city have been more good then harm, as I understand it in terms of internal affairs investigations, but you never know what will happen with this new technology.
And for those that think the cameras should be on continuously, I will tell you that it is impractical. Besides storage and removal issues, the fact of the matter is that officers discuss things in terms of investigations that the public need not know about, ie. speculations, hunches, ideas, tactical plans, etc. Not only will this give information to the public that would potentially be unsafe for officers (tactical planning in future operations), but could cloud issues in court.
Imagine a jury hearing an officer say, "I just know this guy hid the gun around here," to his partner, and lo and behold, he finds a gun. The defense attorney will turn around and say that the officer planted it because he was determined to find his client at fault. Remember Mark Furman's use of the word ni**er was supposedly evidence to his lack of credibility in the O.J. Simpson case.
And besides, who wants to view a cop sitting on the crapper....